A recent look at the U.S. Drought Monitor’s Pennsylvania record shows something that does not match the state’s wet-region reputation: large portions of Central PA have spent meaningful stretches of the last several years in moderate or severe drought categories. The interesting part is not any single dry summer. The interesting part is the pattern — drought episodes arriving close enough together that trees never fully recover between them. That is the setup for the cumulative canopy decline showing up in Centre, Blair, Huntingdon, and Mifflin counties now.
What drought actually does to a tree
A mature tree loses water constantly through transpiration — water pulled up from the roots, moved through the trunk, and released as vapor through leaf stomata. On a hot summer day, a single large shade tree can transpire hundreds of gallons. When the soil runs dry, the tree has two choices: close stomata to conserve water (which also stops photosynthesis) or keep them open and risk embolism — air bubbles forming in the water columns that move up the trunk.
The research suggests that most of the long-term damage from drought comes from embolism events. Once a water-conducting vessel cavitates, many tree species cannot repair it. The tree effectively loses that “straw” for the rest of its life. Enough losses across enough vessels, and the tree can no longer move water to its canopy even when rain returns. USDA Forest Service climate resources describe this as one of the key mechanisms behind delayed drought mortality.
Why the damage does not show up right away
This is the part that confuses most homeowners. A tree does not usually die during a drought. It dies a year or two after. Sometimes three years after. Worth noting: the same pattern appears in forest mortality data — spikes in tree death often trail the drought event by one to four growing seasons.
What’s happening is a cascade: drought weakens the tree, the tree closes stomata and reduces photosynthesis, carbohydrate reserves drop, defensive chemistry against pests and pathogens drops with them, and the tree becomes vulnerable to whatever secondary problem comes next. The secondary problem — bark beetle, canker fungus, root rot — gets the blame on the removal ticket. The real cause was the drought two summers earlier.
The Central PA pattern
The drought data on Central PA is clearer than most residents realize. Extended dry stretches have become a regular feature of the last decade, even in counties that still average healthy annual precipitation. Total rainfall is less diagnostic than timing: a summer with average yearly rainfall but a six-week dry stretch in July and August puts far more stress on trees than a summer with below-average total rainfall distributed evenly.
PA DCNR’s Bureau of Forestry has flagged drought-and-heat stress as a compounding factor in the regional decline of species already under other pressure — ash from emerald ash borer, beech from beech leaf disease, hemlock from woolly adelgid. The drought does not cause the primary problem, but it accelerates every other problem simultaneously.
Which Central PA species show stress first
A useful pattern from ecological literature and Penn State Extension field observation: species native to wetter habitats decline earliest under drought pressure, while species with deep taproots and drought tolerance hold out longer. One thing to flag:
- Declining earliest under drought: yellow birch, sugar maple (on thin soils), eastern hemlock, red maple in wet-site plantings, Norway spruce in full sun.
- Intermediate: white pine, red oak, tulip poplar, American beech.
- Most drought-tolerant: white oak, chestnut oak, bur oak, black walnut on deep soils.
None of this means a drought-tolerant species cannot die from drought. It means the order of decline across a mixed woodlot is predictable, and the sugar maples and hemlocks are usually the early warning system.
Watering a mature tree actually works
Penn State Extension’s drought watering guidance is specific on this point: deep, infrequent watering is far better than frequent shallow watering. For a mature tree under drought stress, a slow soak that delivers one to two inches of water over the root zone, repeated every two to three weeks during a dry stretch, addresses the root zone the tree actually uses.
Practical notes from that guidance:
- Water at the drip line and beyond, not against the trunk. The active feeder roots are out near the edge of the canopy and past it.
- A soaker hose left running for several hours delivers the water slowly enough to soak in rather than run off.
- Mulching the root zone — two to four inches of wood chips, pulled back from the trunk — reduces evaporation by a measurable fraction and moderates soil temperature.
- Newly planted trees (under five years old) need more frequent watering than mature trees during any dry stretch.
What to do this season
For a Central PA property with mature trees that have been through multiple recent dry summers:
- Watch for signs of cumulative stress: earlier fall color than surrounding trees of the same species, smaller leaves, thinner canopies, increased dead tips.
- In any summer that goes three weeks without an inch of rain, plan to deep-water the most stressed trees.
- Do not fertilize drought-stressed trees. Fertilizer pushes growth the tree cannot support.
- Have any tree showing significant decline assessed before the next pest event arrives — drought-stressed trees are the first to fall when a borer or pathogen shows up.
For a mature-tree drought assessment across Centre County, Huntingdon County, and the surrounding region, call (814) 553-0303. Arbor Pro’s Tree Service operates under PA Contractor License PA079160.